The use is well-known of aqueous dispersions of polytetrafluoroethylene (PTFE) for obtaining coatings endowed with a high resistance to heat and to the chemical agents, with good mechanical strength to wear and friction. The use is also well-known of PTFE modified with small amounts of a mostly fluorinated comonomer, as well as that of PTFE blends with other fluorinated polymers, for the purpose of obtaining improved characteristics of impermeability and of flexibility of the coating layer. The essential technical problem in the accomplishment of these coatings is shown by the adhesion to the metal substrate, which must be very high also in view of the temperatures of use of the manufactured articles, which can sometimes be very high.
A good adhesion was obtained with the presence of chromic acid in the aqueous dispersion of the polymer, in particular in combination with phosphoric acid. This system is not allowable if the treated articles are used in the alimentary field, due to the high toxicity of the chromic acid which is present, even if in trace amounts, in the obtained layer of polymeric coating.